Radar sensors are used nowadays in a plurality of applications. Radar sensors can be used, for example in mobile or stationary applications for surroundings monitoring. Radar sensors are used in vehicles, for example, to detect preceding vehicles or ones approaching from behind, and to decelerate the vehicle if applicable or to warn the driver.
Stationary radar sensors, on the other hand, can be used, for example, to monitor a predetermined space. Railway crossings, for example, can be monitored using a radar sensor. The intention is to detect at such railway crossings, with the aid of the radar sensor, whether an object is present, for example, on or in the vicinity of the tracks. If such an object is detected, a warning can be directed to a driver of an approaching train or the train can be, for example, automatically decelerated.
In order to allow effective monitoring of such a predetermined space using stationary radar sensors, it is necessary to be able to determine the size of the objects sensed by the radar sensor.
Patent document US 2 011 007 157 A1 presents, for example, a radar system for monitoring a predetermined space.
Such radar systems may possess a height estimating system for objects located in front of the respective radar sensor. This height estimate requires, however, a parallel alignment of the vertical sensor axis with the plane on which the objects are located. Even small deviations, of a few tenths of a degree, between said plane and the vertical sensor axis can result in large errors in the estimate of the object's size. For example, a small deviation of 1° for a distance of 20 m between the object and the radar sensor already yields an inaccuracy of 0.5 m in the height estimate. This situation is illustrated in FIG. 6.
In FIG. 6 a radar sensor is disposed at a height h. The road, which represents the plane to be monitored, extends horizontally. The vertical sensor axis is rotated upward through an angle α. The object height is consequently estimated by the radar sensor to be lower (crosshatched area) than the actual object height (non-crosshatched area).